Quartzite from the summit of Quandary Peak,

--- Tenmile Range, Colorado, U.S.A.

Figures 1-2.
A white and green quartzite from the summit ridge of Quandary Peak,
elevation 14,265 feet (4,348 metres), the highest point in the Tenmile Range,
south of Breckenridge. Here we see the setting, with a view (left)
westwards from the summit, the sample location in the immediate foreground.
At right is a view towards the east, and the summit, showing quartzite
outcrop.
This month's feature is dedicated to those fifty-odd cheerful souls
who spent much of 21st September 2015 hiking up and down the mountain in perfect
weather!

"Rock of the Month #174, posted for December 2015" ---

A quartzite from the heights of the Rockies,

in central Colorado.
Professor W.H. Brewer of Yale was an expert of his day
on the mountainous regions of the West.
He noted that
"near Mount Lincoln is a peak sometimes called Quandary Peak,
sometimes McCollough's Peak, sometimes Ute Peak, and one man
called it Hoosier Peak" (Brewer, 1872, p.203).
The Tenmile Range is near Breckenridge, with 14,265' Quandary
Peak, Fletcher Mountain and Crystal Peak: nearby is the
Mosquito Range with Mount Lincoln, Mount Democrat and Mount
Bross, all "14,000ers" of which the tallest are
Mount Massive (14,421') and Mount Elbert (14,433').
Ormes (1979) was long a standard reference for the 54
Colorado summits 14,000 feet or more in height:
now guidebooks abound.
Quandary Peak, an east-west ridge on the north
side of the valley of the Blue Lakes, slopes down toward the treeline and
the road through Hoosier Pass to the east
(National Geographic, 2013).
Hikes up Quandary Peak are described by, e.g.,
Mary Ellen Gilliland (2006, pp.18-19) and Gerry Roach (1999,
pp.54-59). The latter includes lists of all 54 Colorado peaks over 14,000 feet
(4,267 metres),
and of 109 over 13,800 feet.

The area of Quandary Peak, southwest of
Breckenridge, is a Proterozoic terrane with granitic rocks in the
1400-1000 Ma age range, intruding 1800-1700 Ma metamorphic host
rocks such as gneiss, schist, migmatite, minor quartzite and
conglomerate. Cambrian quartzite (the Sawatch quartzite)
occurs in the vicinity of Quandary Peak,
and a short distance to the southwest, across the
continental divide around Leadville (GTR Mapping, 2013).

Figure 3.
A fresh broken surface through a sample from the summit ridge.
Tough white, granular, massive quartzite is overlain
along a sharp contact by
a dull green layer of otherwise similar rock.
Sample collected in situ on Monday, 21 September 2015.

So, how old is the quartzite around the summit?
Such clastic sediments can be hard to date, since fossils are
commonly absent. A maximum age can sometimes be set by
direct dating of detrital zircon crystals, washed into the
sandy protolith of the quartzite following erosion of
the original host rock of the zircon, which is typically a
granitic intrusion wherein the zircon crystallized.
Lacking the resources for such advanced study,
a literature search may offer correlations with similar strata
elsewhere. Charles Doolittle Walcott to the rescue!

Walcott (1891) reviewed Cambrian strata and fossils across the
USA and Canada. He noted that around Quandary Peak there is a
unit 150-200 feet thick, with a few inches to one foot of
conglomerate at the base. The lower 100 feet is `finely and rather
thinly bedded white saccharoidal quartzites', the quartzite
succeeded by more shaly layers. There are few or no fossils in the
clastic sediment, but a bed of greenish chloritic slate occurs on the
east side of the peak, about a mile above the Monte Cristo mine,
and the slate contains Dikelocephalus, a trilobite akin to the D.
minnesotensis found in the Potsdam Formation in Wisconsin. Thus
the Quandary Peak quartzite is evidently of Cambrian age, and so
an outlier atop the Precambrian basement.

Pending the preparation of a thin section (Figs. 4-5),
we can speculate that the green
colour seen in one or more layers of the quartzite is the
mineral glauconite. This iron-potassium sheet-silicate mineral,
related to the micas, is quite widely found in sediments of
Cambrian to Recent age. It is decidely uncommon in older rocks,
although numerous reports exist of its occurrence
in the late Proterozoic Vindhyan sediments of central India.
It is known in Cambrian strata across North America, e.g.,
in Quebec, Alberta, South Dakota and Texas, hence
it would not be too surprising to find it in Colorado,
if we accept that this unit is indeed of Cambrian age.
Halka and Chronic (2014, p.145) show the ridge of Quandary as being composed largely
Proterozoic metasediments, with older Paleozoic strata on the east flank,
but they also note (ibid., p.215) that Paleozoic sediments cap the peak,
"tiny remnants of the strata that once arched across the range".

Figures 4-5.
Two photomicrographs of the green layer in the quartzite.
Both images in crossed-polarized transmitted light,
nominal magnification 50X, long-axis field of view 1.7 mm.
The thin section reveals a rock that is largely (circa 94-96%)
fine-grained quartz,
with moderately rounded
grains mostly 0.4-0.8 mm, or as large as 1.2 mm in diameter.
The quartz is partially cemented by a carbonate (4%).
Rare grains of detrital green tourmaline, and
of equant pyrite, are present.
Thin veinlets lined by muscovite mica cut the rock (Fig. 4).
In the green layer thin films of
a fine-grained flakey sheet silicate form a rind around quartz grains,
and occasionally discrete rounded pellets darkened by fine-grained
iron oxides (limonite, hematite). This silicate,
3% of the green layer,
appears to lend its colour to the bulk rock, and
may indeed be glauconite,
but the identification is tentative.
In the photomicrographs the quartz is seen in variable degrees of extinction,
while the fine-grained phases at grain boundaries appear dark.
The rock is a mature, moderately sorted quartzite.

The old Monte Cristo mine, mentioned
above, is situated below tree line,
low on the east flank of the mountain. It is
said to have yielded ore with a modest grade of silver (15 ounces/ton) in
argentiferous galena with sphalerite, hosted by Paleozoic
(Silurian) quartzite (Lakes, 1888, pp.104-105).

In the past half-century,
by far the most famous mine on the east side of the
divide separating Breckenridge from Leadville would be a mineral
specimen mine, the Sweet Home mine west of Alma, on the
east side of the northern Mosquito Range, which is world-famous for
rhodochrosite specimens. The mine and its minerals are
documented in the July 1998 issue of Mineralogical
Record, a multi-author review that goes far beyond the
frequent appearance of exquisite,
shocking-pink carbonate rhombs in the "vanity
press" of minerals!
The range is a gently east-dipping succession to Paleozoic
sediments of Cambrian to Carboniferous age, intruded by a range
of Laramide to Tertiary stocks, dykes and sills (Misantoni et
al., 1998). The Sweet Home workings lie below the level of the Paleozoic sequence,
in Precambrian granitoids cut by Tertiary intrusives.